Abstract:
Time synchronization is an essential service in embedded
wireless networked sensing and control systems. It is used
to enable tasks such as synchronized data sampling and ac-
curate time-of-flight estimation, which can be used to lo-
cate nodes. The deviation in nodes’ knowledge of time
and inter-node resynchronization rate are affected by three
sources of time stamping errors: network wireless communi-
cation delays, platform hardware and software delays, and
environment-dependent frequency drift characteristics of the
clock source. The focus of this work is on the last source of
error, the clock source, which becomes a bottleneck when
either required time accuracy or available energy budget
and bandwidth (and thus feasible resynchronization rate)
are too stringent. Traditionally, this has required the use of
expensive clock sources (such as temperature compensation
using precise sensors and calibration models) that are not
cost-effective in low-end wireless sensor nodes. Since the
frequency of a crystal is a product of manufacturing and
environmental parameters, we describe an approach that
exploits the subtle manufacturing variation between a pair
of inexpensive oscillators placed in close proximity to algo-
rithmically compensate for the drift produced by the en-
vironment. The algorithm effectively uses the oscillators
themselves as a sensor that can detect changes in frequency
caused by a variety of environmental factors. We analyze
the performance of our approach using behavioral models
of crystal oscillators in our algorithm simulation. Then we
apply the algorithm to an actual temperature dataset col-
lected at the James Wildlife Reserve in Riverside County,
California, and test the algorithms on a waveform genera-
tor based testbed. The result of our experiments show that
the technique can effectively improve the frequency stability
of an inexpensive uncompensated crystal 5 times with the
potential for even higher gains in future implementations.